Kinins are endogenous peptides formed in plasma and peripheral tissues in response to tissue injury or infection following catalytic cleavage of kininogens by kallikrein enzymes. Kinins play an important role in the pathophysiological processes accompanying pain and inflammation. Their biological actions are mediated by two G-protein coupled membrane receptors, denoted B1 and B2. Both B1 and B2 receptors have been cloned [Biochem. Biophys. Res. Commun., 184 (1992) 260-268 and J. Biol. Chem., 269 (1994) 21583-21586] and the mechanisms regulating their expression, self-maintenance and signaling function is under intensive investigations [Pharmacol. Rev., 57 (2005) 27-77].
The first set of kinins, bradykinin (BK) and kallidin (LysBK) preferentially act through stimulation of constitutively expressed and rapidly desensitizing B2 receptors, which are widely distributed in many tissues. On the other hand, their active carboxypeptidase metabolites, the second set of kinins, desArg9BK (DABK) and LysdesArg9BK (LysDABK) activate inducible and non-desensitising B1 receptors, which are rarely expressed under non-pathological conditions. Generally B1 receptors rapidly appear after injuries of various natures (tissue trauma, infections, etc.). Thus the B1 receptor up-regulation appears to be part of a generalized response that includes the local co-expression (eventually up-regulation) of enzymes, receptors, autacoids, cytokines and chemokines that notoriously play key roles in the early and late responses of tissues to various types of injury.
In animal models it has been demonstrated that there is a switch in dominance of function from B2 to B1 in chronic inflammatory states. While the B2 receptor is implicated in the acute phase of the inflammatory and pain response, the B1 receptor is involved in the chronic phase of this response. The involvement of kinin receptors in inflammation and pain transduction has been supported by the results of studies on mice lacking bradykinin B1 receptors. B1 receptor deficient mice are different from wild-type mice in sensory functions, exhibiting increased analgesic thresholds to noxious chemical and heat stimuli, and drastic reduction in the accumulation of polymorphonuclear leukocytes at sites of inflammation [PNAS, 97 (2000) 8140-8145 and Neuropharmacology 41 (2201) 1006-1012]. Furthermore the most original, finding in B1 receptor deficient mice was the direct evidence for a role of central kinin receptors in nociception suggesting that the hypoalgesia seen in B1-receptor knockout mice is partly due to reduced central sensitisation in the spinal cord. However, apart from the above changes B1 knockout mice were apparently normal without any apparent pathological changes.
Apart from the evidence of basal expression of B1 receptors on the periphery recently more and more evidence shows that B1 receptors are constitutively expressed ‘centrally’ in some neuronal elements, including the spinal cord and some higher structures as well. The function of these receptors is unclear but they have been implicated in pain transmission and hyperalgesia. Therefore, B1 receptor antagonists are believed to be useful in alleviating pain not only via peripheral sites but also to have possibly broader spectrum of analgesic effects if they block central B1 receptors as well [NeuroReport 11 (2000) 4003-4005; NeuroReport, 12 (2001) 2311-2313; Neuroscience 107 (2001) 665-673 and Neuroscience Letters 294 (2000) 175-178].
On the basis of scientific data bradykinin receptors are involved in mediation of pain and hyperalgesia in several ways. B1 receptor antagonists may have diverse modes of action. They have (1) indirect (‘peripheral’) effects on the nociceptors via inhibition of release of other algogenic mediators (prostaglandins, cytokines and nitric oxide) from cells other than sensory neurones (macrophages, fibroblasts or endothelial cells); (2) direct (‘peripheral’) effects on nociceptors expressing B1 receptors (constitutively) or upon induction and (3) ‘central’ effects on pain processing in the superficial dorsal horn of spinal cord.
Therefore, an orally active non-peptide bradykinin B1 receptor antagonist could be a potential therapeutic agent in the treatment of chronic inflammatory pain.